  
  [1X8 [33X[0;0YOptions Stack[133X[101X
  
  [33X[0;0Y[5XGAP[105X supports a [13Xglobal options system[113X. This is intended as a way for the user
  to  provide  guidance  to  various  algorithms  that  might  be  used  in  a
  computation.   Such   guidance   should   not   change   mathematically  the
  specification of the computation to be performed, although it may change the
  algorithm  used.  A  typical  example is the selection of a strategy for the
  Todd-Coxeter coset enumeration procedure. An example of something not suited
  to  the  options  mechanism  is  the  imposition  of  exponent  laws  in the
  [22Xp[122X-Quotient algorithm.[133X
  
  [33X[0;0YThe  basis  of  this system is a global stack of records. All the entries of
  each record are thought of as options settings, and the effective setting of
  an  option  is  given  by  the topmost record in which the relevant field is
  bound.[133X
  
  [33X[0;0YThe reason for the choice of a stack is the intended pattern of use:[133X
  
  [33X[0;0Y[10XPushOptions( rec( [3Xstuff[103X[10X ) );[110X[133X
  
  [33X[0;0Y[10XDoSomething( [3Xargs[103X[10X );[110X[133X
  
  [33X[0;0Y[10XPopOptions();[110X[133X
  
  [33X[0;0YThis  can  be  abbreviated,  to  [10XDoSomething(  [3Xargs[103X[10X  : [3Xstuff[103X[10X );[110X with a small
  additional abbreviation of [3Xstuff[103X permitted. See [14X4.11-2[114X for details. The full
  form  can be used where the same options are to run across several calls, or
  where  the  [10XDoSomething[110X  procedure  is  actually an infix operator, or other
  function with special syntax.[133X
  
  [33X[0;0YAn alternative to this system is the use of additional optional arguments in
  procedure  calls.  This  is not felt to be sufficient because many procedure
  calls  might  cause, for example, a coset enumeration and each would need to
  make  provision  for  the possibility of extra arguments. In this system the
  options  are  pushed  when the user-level procedure is called, and remain in
  effect (unless altered) for all procedures called by it.[133X
  
  [33X[0;0YNote  that  in some places in the system optional records containing options
  which  are  valid only for the immediate function or method call are in fact
  used.[133X
  
  
  [1X8.1 [33X[0;0YFunctions Dealing with the Options Stack[133X[101X
  
  [1X8.1-1 PushOptions[101X
  
  [29X[2XPushOptions[102X( [3Xoptions[103X, [3Xrecord[103X ) [32X function
  
  [33X[0;0YThis  function pushes a record of options onto the global option stack. Note
  that [10XPushOptions( rec( [3Xopt[103X[10X:= fail ) )[110X has the effect of resetting the option
  [3Xopt[103X,  since an option that has never been set has the value [9Xfail[109X returned by
  [2XValueOption[102X ([14X8.1-5[114X).[133X
  
  [33X[0;0YNote that there is no check for misspelt or undefined options.[133X
  
  [1X8.1-2 PopOptions[101X
  
  [29X[2XPopOptions[102X(  ) [32X function
  
  [33X[0;0YThis  function removes the top-most options record from the options stack if
  there is one.[133X
  
  [1X8.1-3 ResetOptionsStack[101X
  
  [29X[2XResetOptionsStack[102X(  ) [32X function
  
  [33X[0;0Yunbinds (i.e. removes) all the options records from the options stack.[133X
  
  [33X[0;0Y[13XNote:[113X  [2XResetOptionsStack[102X  should [13Xnot[113X be used within a function. Its intended
  use  is  to  clean  up the options stack in the event that the user has [9Xquit[109X
  from   a  [9Xbreak[109X  loop,  so  leaving  a  stack  of  no-longer-needed  options
  (see [14X6.4-1[114X).[133X
  
  [1X8.1-4 OnQuit[101X
  
  [29X[2XOnQuit[102X(  ) [32X function
  
  [33X[0;0Ycalled  when  a  user selects to [10Xquit;[110X a break loop entered via execution of
  [2XError[102X ([14X6.6-1[114X). As [5XGAP[105X starts up, [2XOnQuit[102X is defined to do nothing, in case an
  error  is  encountered  during [5XGAP[105X start-up. Here we redefine [2XOnQuit[102X to do a
  variant  of  [2XResetOptionsStack[102X  ([14X8.1-3[114X) to ensure the options stack is empty
  after a user quits an [2XError[102X ([14X6.6-1[114X)-induced break loop. ([2XOnQuit[102X differs from
  [2XResetOptionsStack[102X  ([14X8.1-3[114X)  in  that  it warns when it does something rather
  than  the  other  way  round.)  Currently,  [2XOnQuit[102X  is not advertised, since
  exception handling may make it obsolete.[133X
  
  [1X8.1-5 ValueOption[101X
  
  [29X[2XValueOption[102X( [3Xopt[103X ) [32X function
  
  [33X[0;0YThis  function  is a method for accessing the options stack without changing
  it;  [3Xopt[103X  should  be  the name of an option, i.e. a string. A function which
  makes  decisions that might be affected by options should examine the result
  of [2XValueOption[102X. If [3Xopt[103X is currently not set then [9Xfail[109X is returned.[133X
  
  [1X8.1-6 DisplayOptionsStack[101X
  
  [29X[2XDisplayOptionsStack[102X(  ) [32X function
  
  [33X[0;0YThis function prints a human-readable display of the complete options stack.[133X
  
  [1X8.1-7 InfoOptions[101X
  
  [29X[2XInfoOptions[102X[32X info class
  
  [33X[0;0YThis  info  class can be used to enable messages about options being changed
  (level 1) or accessed (level 2).[133X
  
  
  [1X8.2 [33X[0;0YOptions Stack – an Example[133X[101X
  
  [33X[0;0YThe  example  below  shows  simple  manipulation of the Options Stack, first
  using  [2XPushOptions[102X ([14X8.1-1[114X) and [2XPopOptions[102X ([14X8.1-2[114X) and then using the special
  function calling syntax.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xfoo := function()[127X[104X
    [4X[25X>[125X [27XPrint("myopt1 = ", ValueOption("myopt1"),[127X[104X
    [4X[25X>[125X [27X      " myopt2 = ",ValueOption("myopt2"),"\n");[127X[104X
    [4X[25X>[125X [27Xend;[127X[104X
    [4X[28Xfunction(  ) ... end[128X[104X
    [4X[25Xgap>[125X [27Xfoo();[127X[104X
    [4X[28Xmyopt1 = fail myopt2 = fail[128X[104X
    [4X[25Xgap>[125X [27XPushOptions(rec(myopt1 := 17));[127X[104X
    [4X[25Xgap>[125X [27Xfoo();[127X[104X
    [4X[28Xmyopt1 = 17 myopt2 = fail[128X[104X
    [4X[25Xgap>[125X [27XDisplayOptionsStack();[127X[104X
    [4X[28X[ rec([128X[104X
    [4X[28X      myopt1 := 17 ) ][128X[104X
    [4X[25Xgap>[125X [27XPopOptions();[127X[104X
    [4X[25Xgap>[125X [27Xfoo();[127X[104X
    [4X[28Xmyopt1 = fail myopt2 = fail[128X[104X
    [4X[25Xgap>[125X [27Xfoo( : myopt1, myopt2 := [Z(3),"aardvark"]);[127X[104X
    [4X[28Xmyopt1 = true myopt2 = [ Z(3), "aardvark" ][128X[104X
    [4X[25Xgap>[125X [27XDisplayOptionsStack();[127X[104X
    [4X[28X[  ][128X[104X
    [4X[25Xgap>[125X [27X[127X[104X
  [4X[32X[104X
  
